I've been hearing rumors that the fairy type was to counter the dark type...The rumoured Fairy type is strong against Dragon, right?
Dark/Dragon for Yveltal and Fairy/Grass for Xerneas perhaps? Yveltal will likely be learning a ton of Flying moves judging by it's appearance, so it would balance it out a bit. Now I know what you're thinking: "But the tao trio wasn't balanced either!" - Yes, I know. But it could still work, considering purely type-wise, Fairy/Grass and Dark/Dragon ISN'T balanced.